What it measures

A percolation ("perc") test digs one or more holes to the depth a septic drain field would sit at, fills them with water, and times how fast the water level drops once the soil is saturated. That rate - minutes per inch - determines whether a standard drain field can absorb wastewater fast enough, and how large the field needs to be if it can.

Who performs it, and when

Most jurisdictions require a licensed soil evaluator, engineer, or the local health department itself to either run or witness the test before a septic permit is issued. It's typically ordered after a buyer is under contract on raw land, or before a builder finalizes a septic design - not something a homeowner runs informally, since the result has to be filed with the permit application.

Why a county-level rating like this site's isn't a substitute

USDA's SSURGO data is built from soil-scientist mapping at a scale meant for planning and comparison, generalized across a whole survey area. A perc test measures the specific hole, on the specific lot, at the specific depth a system would actually use. Two lots in a county rated the same way can still perc differently - the county figure is what to expect walking in, not what a permit application will accept.

If a lot fails

A failed perc test doesn't necessarily end a project. It usually means an engineered alternative - a mound system, an aerobic treatment unit, or a different field design - rather than a conventional gravity system. That's the gap this site's cost figures are pointing at: roughly $5,500 for a conventional system versus $17,500 typical for an engineered one.

Where to go next

Real counties this plays out on: Sutton County, TX, Stewart County, TN, Sheridan County, KS.

Conventional vs. engineered septic systems · Buying rural land? What to check before you rely on septic